Light receiving element and method of manufacturing the same

ABSTRACT

A light receiving element for converting a received light signal into an electric signal and its manufacturing method are disclosed. The light receiving element includes a semiconductor substrate of a first conduction type; a semiconductor layer of a second conduction type; and a photo-absorption layer interposed between the semiconductor substrate and the semiconductor layer of the second conduction type. The semiconductor substrate comprises: a first groove having an inclination with respect to an incidence plane of the light signal so that the light signal can be refracted when the light signal has been incident to the first groove; and a second groove by which the light signal having been refracted by the first groove is reflected fully and then absorbed into the photo-absorption layer, so that a vertical-incidence drift of the light signal toward the photo-absorption layer is minimized.

CLAIM OF PRIORITY

This application claims priority to an application entitled “Lightreceiving element and method of manufacturing the same,” filed in theKorean Intellectual Property Office on Mar. 28, 2003 and assigned Ser.No. 2003-19621, the contents of which are hereby incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a light receiving element forconverting a received light signal into an electric signal and a methodof manufacturing the same.

2. Description of the Related Art

An optical coupling allows light signals emitted from light sources,such as a laser diode, fiber, a Planar Lightwave Circuit (PLC) deviceand the like, to arrive at the light receiving surface without a loss,so as to be converted into optimal electric signals. Many studies haveshown that a vertical photodiode has a higher reliability than awaveguide photodiode for the provision of light signal conversion.

In order to manufacture a light module of ultra-low cost, the lightmodule must be manufactured in complete automatization, that is, in achip mounting method. Therefore, two-dimensional optical coupling isnecessary throughout the field of optical coupling, such as opticalcoupling between a laser diode and a photodiode, between a fiber and aphotodiode, between a PLC and a photodiode, and so forth.

FIG. 1 is a sectional view showing the structure of a photo detector fortwo-dimensional optical coupling according to the prior art. The photodetector is a light receiving element having the so-callededge-illuminated refracting-facet structure.

As shown, the photo detector includes: an InP substrate 1, alight-incidence plane 2, an n-type InP layer 3, a photo-absorption layer4, a p-type InP layer 5, a p-type electrode 6, and an n-type electrode7. The light-incidence plane 2 of the photo detector is formed so as tobe inclined at an angle of θ through a wet etching process. As a result,the photo detector has a structure in which incident light is refractedto the photo-absorption layer 4. The refracted light, which is incidentto the photo-absorption layer, has longer effective absorption lengththan that of the light being incident in a vertical direction, thusincreasing the receiving sensitivity.

However, the conventional photo detector undergoes a chemical etchingprocess for forming an angled facet. As such, the manufacturing of thephoto detector according to the conventional art has drawbacks in thatreproducibility and uniformity of elements are difficult to achieve.Furthermore, if an anti-reflective coating layer is implemented toimprove the performance, a difficult task of mesa etching is requiredwhich in turn reduces the productive yield as it requires additionalsteps.

SUMMARY OF THE INVENTION

Accordingly, the present invention has been made to overcome theabove-mentioned problems and provides additional advantages, byproviding a light receiving element having a vertical structure toenable two-dimensional coupling of a light signal generated from a lightsource.

One aspect of the present invention is to provide a light receivingelement that may be realized in a reliable and simple implementation.

In one embodiment, a light receiving element for receiving a lightsignal and converting the received light signal into an electric signalis provided and includes: a semiconductor substrate of a firstconduction type; a semiconductor layer of a second conduction type; and,a photo-absorption layer interposed between the semiconductor substrateand the semiconductor layer of the second conduction type. Thesemiconductor substrate further includes: a first groove having aninclination with respect to an incidence plane of the light signal sothat the light signal can be refracted when the light signal has beenincident to the first groove; and a second groove by which the lightsignal having been refracted by the first groove is totally reflectedand then absorbed into the photo-absorption layer, so that avertical-incidence drift of the light signal toward the photo-absorptionlayer is minimized.

It is preferred that the semiconductor substrate is made from asemiconductor material in which a specific crystalline direction isetched more slowly than other directions when it is wet-etched, so thatthe semiconductor substrate can achieve an inclined profile after beingetched. The semiconductor substrate may be made from one of a group VI,a group II-VI, and a group III-V semiconductor substrate.

It is also preferred that the first groove and the second groove areformed so as to have a slant angle of 50° to 60° on the basis of ahorizontal direction of the semiconductor substrate, and are formed soas to have a ‘U’ shape or a ‘V’ shape.

It is also preferred that the total reflection layer is made from one ofan air layer, a vapor layer, and a metal layer having a thickness largerthan the skin depth of the metal.

In another embodiment, a method of manufacturing a light receivingelement is provided by performing the following steps: growing a firstsemiconductor layer, a photo-absorption layer, and a secondsemiconductor layer on a semiconductor substrate of a first conductiontype, each of the first semiconductor layer and the second semiconductorlayer having the same conduction type as that of the first semiconductorlayer; selectively converting the second semiconductor layer from thefirst conduction type to a second conduction type by diffusingimpurities; and, forming a first groove and a second groove bywet-etching the semiconductor substrate. The method further comprises astep of forming an anti-reflective coating layer on the first groove anda step of forming a total reflection layer made of metal on the secondgroove.

BRIEF DESCRIPTION OF THE DRAWINGS

The above features and advantages of the present invention will be moreapparent from the following detailed description taken in conjunctionwith the accompanying drawings, in which:

FIG. 1 is a sectional view showing the structure of a photo detectorhaving an edge-illuminated refracting-facet according to the prior art;

FIG. 2 is a sectional view showing the structure of a light receivingelement according to a preferred embodiment of the present invention;

FIG. 3 is a view for explaining the characteristics of absorptioncoefficients according to the wavelengths in various semiconductors;

FIG. 4 is a view for explaining Snell's law;

FIG. 5 is a view for explaining the principle of total reflection;

FIG. 6 is a sectional view showing the structure of a light receivingelement according to another embodiment of the present invention; and,

FIGS. 7 a to 7 e are sectional views for showing the manufacturingprocess of a light receiving element according to a preferred embodimentof the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Hereinafter, a light receiving element and a method of manufacturing thesame according to preferred embodiments of the present invention will bedescribed with reference to the accompanying drawings. It is to be notedthat the same elements are indicated with the same reference numeralsthroughout the drawings. For the purposes of simplicity and clarity, adetailed description of well known functions and configurationsincorporated herein will be omitted as it may make the subject matter ofthe present invention unclear.

Referring to FIG. 2, a light receiving element 100 according to apreferred embodiment of the present invention includes an n-type InPsubstrate (first semiconductor layer) 110, an InGaAs photo-absorptionlayer 120 formed on the substrate 110, an InP window layer 130 formed onthe photo-absorption layer 120, p-type InP active region (secondsemiconductor layer) 140 formed in appointed region of the window layer130, a passivation layer 150 formed on the window layer 130, a p-typeelectrode 160 formed on the active region, an SiN_(x) anti-reflectivecoating layer 170 formed on the bottom of the substrate 110, a totalreflection layer 180, and a n-type electrode 190 formed on the bottom ofthe substrate 110.

The semiconductor substrate 110 consists of a semiconductor materialthat (Miller-Index: 111) is etched slowly so that an inclined profilecan be formed after the etching process. The etching process isperformed with a wet solution in which a specific crystalline directionis etched more slowly than other directions in accordance withcrystallography process. The semiconductor substrate 110 is made of oneof a group VI, a group II-VI, and a group III-V semiconductor substratesand produced in a single crystal growth using a chemical vapordeposition process.

The photo-absorption layer 120 is made of materials having a smallerbandgap energy than that of a wavelength of light signals intended forabsorption thereon. To this end, InGaAs may be used to make thephoto-absorption layer 120. In contrast, the window layer 130 is made ofmaterials having greater bandgap energy than that of a wavelength oflight signals which is intended for absorption, and InP may be used tomake the window layer 130. Note that window layer 130 does not absorblight but passes the light passing therethrough. Therefore, The windowlayer 130 is consisted of a larger bandgap energy.

The active region 140 performs the function of converting light signalsabsorbed through the photo-absorption layer 120 into electric signals,and is formed by selectively diffusing impurities having a conductiontype which is opposite to the conduction type of the impurities in thesubstrate 110. The active region 140 is formed by diffusing impuritiesonly in a specific region through a photolithograph process.

The passivation layer 150 prevents oxidation of the boundary surface ofthe window layer 130 positioned under the passivation layer 150 and maybe made using dielectric materials, such as a silicon nitride.

The p-type electrode 160 and the n-type electrode 190 detects electricsignals, which are photoelectric-converted in the active region 140,using an external circuit, and maybe made of metal materials.

The anti-reflective coating layer 170 enables light signals, which areinputted from light sources such as a laser diode, fiber, a PLC (PlanarLightwave Circuit) device and the like, to go through the inside of thesubstrate 110 without reflection. The anti-reflective coating layer 170may be formed by depositing anti-reflective materials on a first groovesurface A, which is formed with an inclination through an etchingprocess. In an alternate embodiment, the anti-reflective coating layer170 may be omitted, and in this case, about 30 to 35% of incident lightsignals are reflected off. Therefore, it is determined whether or notthe anti-reflective coating layer 170 is need depending on thereflection (that is, the degree of light loss), the convenience of themanufacturing process, and the characteristic of a light element. Forexample, in the case of an MPD (Monitor Photo Diode) performing amonitoring function of light signals, it is preferred not to form ananti-reflective coating layer 170 for the convenience of a manufacturingprocess.

The total reflection layer 180 reflects all light signals inputted intothe inside of the substrate 110 through the anti-reflective coatinglayer 170. The total reflection layer 180 may consist of an air layer ora vapor layer, without other material layers, on second groove surfaceB, which is formed with an inclination on a rear surface of thesubstrate 110 by an etching process. Note that air layer does notperform any process. Further, the total reflection layer 180 may beformed by depositing total-reflection materials on the second groovesurface B through a CVD (Chemical Vapor Deposition) process or a PVD(Physical Vapor Deposition) process. For example, the total-reflectionmaterials can use all materials having reflective index less than 2.7.

Now, the operation of the light receiving element having theconstruction as described above is as follows.

Referring again to FIG. 2, light signals, which are inputted from lightsources, such as a laser diode, fiber, a Planar Lightwave Circuit (PLC)device and the like, arrive at the first groove surface A, then progressinto the inside of the n-type InP substrate 110 via the anti-reflectivecoating layer 170, which is formed on the first groove surface A. Atthis point, light signals passes through without being absorbed in then-type InP substrate 110. Note that the light signals have wavelengthsof 1.3 μm (energy bandgap of IeV) and 1.55 μm (energy bandgap of 0.8eV), which are common light signals used in most optical communicationsystems. As shown in FIG. 3, the reason for this is that the energybandgap of the InP is too large in normal temperature to absorb anyenergy, thus the light signals pass through the n-type InP substrate110. Therefore, incident light progresses through the first groove A ata refracted state without any light loss. Note that incident light isrefracted whenever it passes two different media from each other, whichis illustrated in Snell's law in which the degree of the refraction oflight is defined when light passes a boundary surface between two mediahaving different properties.

Referring to FIG. 4, Snell's law is defined as:n ₁ sin θ₁ =n ₂ sin θ₂

Herein, n₁ represents a refractive index of an incidence layer throughwhich light is incident to an interface, θ₁ represents an incidenceangle of the light with respect to the vertical line to the interface,n₂ represents a refractive index of a refraction layer through which thelight proceeds after passing the interface, and θ₂ represents arefraction angle of the light with respect to the vertical line to theinterface.

In view of Snell's law, the incident light is refracted when the lightis incident from air (refractive index=1) to the anti-reflective coatinglayer 170 (refractive index of SiN_(x)=2.0), and is also refracted whenthe light is incident from the anti-reflective coating layer 170 to thesubstrate 110 (refractive index of InP=3.47). As such, if theanti-reflective coating layer 170 is formed as multiple layers, theincident light will be refracted as many times as there are layers.

The incident light (θ₂=25.7) progressing inside the substrate 110 isrefracted totally at the second groove surface B having atotal-refraction layer, and thus is incident to the photo-absorptionlayer 120 without loss of light signals. The principle of the totalrefraction of the second groove surface B is as follows.

Referring to FIG. 5, in a case where light is incident from a firstmedium (refractive index of the InP substrate=3.47) having a refractiveindex to a second medium (refractive index of air=1) having a smallerrefractive index than that of the first medium, a refraction angle ofthe light, like the first light shown in FIG. 5, become larger than theincidence angle of the light according to Snell's law. By increasing theincidence angle continuously, when the incidence angle become a criticalangle (θ_(c)), like the second light shown in FIG. 5, the refractionangle become 90°. Further, when the incidence angle become larger thanthe critical angle (θ_(c)), like the third light shown in FIG. 5, allthe light is not refracted, but reflected instead.

In the above scenario, Snell's law is applied as follows.n ₁ sin θ_(c) =n ₂ sin 90°sin θ_(c) =n ₂ /n ₁

Accordingly, since sin θ_(c)=n_((air))/n_((InP)), the critical angle(θ_(c)) is 16.7° and θ₄ is 29°. Therefore, the light incidented on thesecond groove surface B is incident at an angle of 61° (90°−29°=61°) onthe basis of a vertical line of the boundary surface in which the angleof 61° is larger value than the critical angle of 16.7°. As a result,the impinged light is not refracted but totally reflected at the secondgroove surface B. In practice, the vertical-incidence angle of light,which is reflected on the second groove B and then progresses to thephoto-absorption layer 120, deviates very slightly from the central axisto 97° to 94°.

FIG. 6 is a sectional view showing the structure of a light receivingelement according to another embodiment of the present invention. Asshown, the construction and operation of the this embodiment areessentially the same as those described above with respect to FIG. 2.The only notable difference is that a metal layer is provided as thetotal reflection layer. Hence, the discussion of similar componentsdescribed in the preceding paragraphs is omitted to avoid redundancy, asthey are described with respect to FIG. 2.

The metal layer 200 is formed on the second groove surface B, so thatlight signals are reflected totally on the metal layer 200. As the metallayer has a skin depth of about 30 Å to 60 Å depending on the kind ofmetal and the wavelengths, the metal layer 200 is formed so as to have athickness larger than the skin depth.

FIG. 7 a to 7 e are sectional views showing the manufacturing process ofa light receiving element according to a preferred embodiment of thepresent invention.

First, as shown in FIG. 7 a, an InP buffer layer (not shown), aphoto-absorption layer 120, and a window layer 130 are formed insequence, through a single crystal growth of the n-type InP substrate110 using a Metal-Organic Chemical Vapor Deposition (MOCVD) process. Thephoto-absorption layer 120 is made of materials having smaller energybandgap than that of the wavelengths of light signals to be absorbed. Inparticular, InGaAs may be used to make the photo-absorption layer 120.The window layer 130 is made of materials having larger energy bandgapthan that of the wavelengths of light signals to be absorbed. To thisend, InP may be used to make the window layer 130.

As shown in FIG. 7 b, a p-type InP active region 140 is formed bydiffusing p-type impurities selectively in a predetermined region of thewindow layer 130, and a passivation layer 150 is formed from dielectricmaterials so as to prevent oxidation of the interface of the windowlayer 130. Finally, a p-type electrode 160 is formed on the activeregion 140.

Next, as shown in FIG. 7 c, a thinning process of grinding the InPsubstrate 110 to a desired thickness is performed, and etching masks 210are selectively formed on one end of the InP substrate 110. A firstgroove area 220 and a second groove area 230 are determined by theetching masks 210, then the etching masks 210 are formed so that thefirst groove area 220 forms a portion of an edge of one side of the InPsubstrate 110. The etching masks 210 may be made of dielectric filmssuch as SiN_(x), SiO₂ or PR (Photo Resist).

Thereafter, the InP substrate 110 is etched using a wet etching process.An etching solution may be changed depending on the type of substrate.The wet etching makes use of the characteristic that (111) plane isetched more slowly than other directions than (111) plane in the crystalstructure. Therefore, the wet etching is performed until the (111) planeappears. For example, if an InP substrate 110 is used, an HCl-based, anHBr-based or a Br-Me(OH)-based etching solution is used to form the(111) plane. Alternatively, (111) plane may be obtained by using aKOH-based etching solution if an Si substrate, or using a H₂SO₄-basedetching solution of a GaAs substrate. Slant angles of (111) planesformed by such a process are different according to the type ofsubstrate's materials and etching solution employed, but most of the(111) planes have a slant angle of 54.7±55° on the basis of a horizontaldirection. After a first groove A and a second groove B are formedthrough the wet etching process as shown in FIG. 7 d, the etching masks210 are removed (see FIG. 7D).

Finally, as shown in FIG. 7 e, an anti-reflective coating (ARC) layer170 is formed on the first groove A using a Plasma Enhanced ChemicalVapor Deposition (PECVD) or a Sputter technique, forms N-metal 190through sputtering or e-beam evaporation after opening N-type electroderegion through photolithography process, and then a n-type electrode190. Then, the anti-reflective coating layer 170 is deposited to athickness and has a composition capable of achieving an anti-reflectivecondition so that incident light is neither reflected nor lost.

As described above, in a case where the slant angle of the V-groove is55°, the light receiving element according to the present invention canimprove a vertical-incidence angle of light signals toward thephoto-absorption layer by 97° to 94° through refraction and reflectionin the substrate. Therefore, loss of light can be minimized, and it hasan effect in which process the margin is greatly improved since avertical-incidence drift representing the deviation degree of lightsignals according to the thickness of substrates is very small. Inaddition, in accordance with the present invention, a light receivingelement having a vertical structure, which cannot be constructed into atwo-dimensional package in the prior art, can be constructed into atwo-dimensional package. Therefore, in the work for optical coupling,the degree of freedom is reduced from three to two, and thus the workingerror is reduced.

Furthermore, with the manufacturing method of the light receivingelement according to the present invention, an anti-reflective coatinglayer can be formed on a groove surface by using a simple PECVD process,so that it has an effect of increasing its process yield.

While the invention has been shown and described with reference tocertain preferred embodiments thereof, it will be understood by thoseskilled in the art that various changes in form and details may be madetherein without departing from the spirit and scope of the invention asdefined by the appended claims. For example, the technical idea, inwhich the path of light signals is changed by refracting and reflectinglight signals in using a first and a second groove formed on asubstrate, may be variously applied to receiving elements manufacturedin using a group VI, a group II-VI or a group III-V substrate.Therefore, this invention is not to be unduly limited to the embodimentset forth herein, but to be defined by the appended claims andequivalents thereof.

1. A light receiving element for converting a light signal into anelectric signal, comprising: a semiconductor substrate; a semiconductorlayer; and a photo-absorption layer interposed between the semiconductorsubstrate and the semiconductor layer, the semiconductor substratecomprises: a first groove having an inclination with respect to anincidence plane of the light signal so that the light signal can berefracted when the light signal has been incident on the first groove;and a second groove for reflecting the light signal refracted by thefirst groove to be absorbed into the photo-absorption layer, so that avertical-incidence drift of the light signal toward the photo-absorptionlayer is minimized.
 2. The light receiving element of claim 1, whereinthe semiconductor substrate is made from a semiconductor material inwhich a specific crystalline direction is etched slowly, so that thesemiconductor substrate has an inclined profile after being wet-etched.3. The light receiving element of claim 2, wherein the semiconductorsubstrate exposes (111) plane after being etched by a wet solution. 4.The light receiving element of claim 3, wherein the semiconductorsubstrate is made from one of a group VI, a group II-VI, and a groupIII-V semiconductor substrate.
 5. The light receiving element of claim1, wherein the first groove and the second groove are formed to have aslant angle of 50° to 60° relative to a horizontal orientation.
 6. Thelight receiving element of claim 1, wherein the first groove and thesecond groove have a ‘U’ shape or a ‘V’ shape.
 7. The light receivingelement of claim 1, wherein the first groove further comprises ananti-reflective coating layer so that the light signal is refractedwithout a reflection when the light signal is incident thereto.
 8. Thelight receiving element of claim 7, wherein the anti-reflective coatinglayer is a deposited film formed by a chemical vapor deposition processor a physical vapor deposition process.
 9. The light receiving elementof claim 1, wherein the second groove further comprises a totalreflection layer.
 10. The light receiving element of claim 9, whereinthe total reflection layer is made from a metal layer having a thicknesssubstantially greater than the skin depth of the metal layer.
 11. Thelight receiving element of claim 10, further comprising a dielectricfilm formed between the semiconductor substrate and the metal layer. 12.The light receiving element of claim 1, wherein the semiconductorsubstrate has a higher energy band gap than that of the light signal.13. The light receiving element of claim 1, wherein the light receivingelement further includes: a first electrode formed on the semiconductorlayer; and a second electrode formed on a portion of a rear surface ofthe semiconductor substrate.
 14. A method of manufacturing a lightreceiving element, the method comprising the steps of: growing a firstsemiconductor layer, a photo-absorption layer, and a secondsemiconductor layer on a semiconductor substrate of a first conductiontype in sequence, each of the first semiconductor layer and the secondsemiconductor layer having the same conduction type as that of the firstsemiconductor layer; selectively converting the second semiconductorlayer from the first conduction type to a second conduction type bydiffusing impurities; and forming a first groove and a second groove bywet-etching the semiconductor substrate.
 15. The method of claim 14,wherein the semiconductor substrate is one of a group VI, a group II-VI,and a group III-V semiconductor substrate, the semiconductor substratebeing made from a semiconductor material in which a specific crystallinedirection is etched slowly, so that the semiconductor substrate has aninclined profile after being wet-etched.
 16. The method of claim 15,wherein the semiconductor substrate is one of an InP substrate, asilicon substrate, and a GaAs substrate.
 17. The method of claim 16,wherein an etching solution for forming the first groove and the secondgroove is capable of exposing the (111) plane of the semiconductorsubstrate.
 18. The method of claim 14, further comprising a step offorming an anti-reflective coating layer on the first groove.
 19. Themethod of claim 18, wherein the anti-reflective coating layer is formedby a chemical vapor deposition process or a physical vapor depositionprocess.
 20. The method of claim 14, further comprising a step offorming a total reflection layer made from metal materials on the secondgroove.
 21. The method of claim 20, wherein the metal layer is formedhave a thickness substantially greater than that of its skin depth. 22.The method of claim 20, further comprising a step of forming adielectric film between the semiconductor substrate and the metal layer.23. The method of claim 14, further comprising steps of: forming anelectrode of a second conduction type on the second semiconductor layerof the second conduction type; and forming an electrode of a firstconduction type on a portion of a rear surface of the semiconductorsubstrate.